Date:	Wed, 1 Sep 2010 09:21:36 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	linux-mm@...ck.org,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>
Subject: Re: [PATCH 2/5] memcg: quick memcg lookup array

On Tue, 31 Aug 2010 12:44:14 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-08-25 17:07:41]:
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > 
> > Now, memory cgroup has an ID per cgroup and make use of it at
> >  - hierarchy walk,
> >  - swap recording.
> > 
> > This patch is for making more use of it. The final purpose is
> > to replace page_cgroup->mem_cgroup's pointer to an unsigned short.
> > 
> > This patch caches a pointer of memcg in an array. By this, we
> > don't have to call css_lookup() which requires radix-hash walk.
> > This saves some amount of memory footprint at lookup memcg via id.
> > 
> > Changelog: 20100825
> >  - applied comments.
> > 
> > Changelog: 20100811
> >  - adjusted onto mmotm-2010-08-11
> >  - fixed RCU related parts.
> >  - use attach_id() callback.
> > 
> > Changelog: 20100804
> >  - fixed description in init/Kconfig
> > 
> > Changelog: 20100730
> >  - fixed rcu_read_unlock() placement.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> >  init/Kconfig    |   10 +++++++
> >  mm/memcontrol.c |   75 +++++++++++++++++++++++++++++++++++++++++---------------
> >  2 files changed, 65 insertions(+), 20 deletions(-)
> > 
> > Index: mmotm-0811/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0811.orig/mm/memcontrol.c
> > +++ mmotm-0811/mm/memcontrol.c
> > @@ -195,6 +195,7 @@ static void mem_cgroup_oom_notify(struct
> >   */
> >  struct mem_cgroup {
> >  	struct cgroup_subsys_state css;
> > +	int	valid; /* for checking validness under RCU access.*/
> >  	/*
> >  	 * the counter to account for memory usage
> >  	 */
> > @@ -294,6 +295,29 @@ static bool move_file(void)
> >  					&mc.to->move_charge_at_immigrate);
> >  }
> > 
> > +/* 0 is unused */
> > +static atomic_t mem_cgroup_num;
> > +#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> > +static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> > +
> > +/* Must be called under rcu_read_lock */
> > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +{
> > +	struct mem_cgroup *mem;
> > +	/* see mem_cgroup_free() */
> > +	mem = rcu_dereference_check(mem_cgroups[id], rcu_read_lock_held());
> > +	if (likely(mem && mem->valid))
> > +		return mem;
> > +	return NULL;
> > +}
> > +
> > +static void register_memcg_id(struct mem_cgroup *mem)
> > +{
> > +	int id = css_id(&mem->css);
> > +	rcu_assign_pointer(mem_cgroups[id], mem);
> > +	VM_BUG_ON(!mem->valid);
> > +}
> > +
> >  /*
> >   * Maximum loops in mem_cgroup_hierarchical_reclaim(), used for soft
> >   * limit reclaim to prevent infinite loops, if they ever occur.
> > @@ -1847,18 +1871,7 @@ static void mem_cgroup_cancel_charge(str
> >   * it's concern. (dropping refcnt from swap can be called against removed
> >   * memcg.)
> >   */
> > -static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
> > -{
> > -	struct cgroup_subsys_state *css;
> > 
> > -	/* ID 0 is unused ID */
> > -	if (!id)
> > -		return NULL;
> > -	css = css_lookup(&mem_cgroup_subsys, id);
> > -	if (!css)
> > -		return NULL;
> > -	return container_of(css, struct mem_cgroup, css);
> > -}
> > 
> >  struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
> >  {
> > @@ -1879,7 +1892,7 @@ struct mem_cgroup *try_get_mem_cgroup_fr
> >  		ent.val = page_private(page);
> >  		id = lookup_swap_cgroup(ent);
> >  		rcu_read_lock();
> > -		mem = mem_cgroup_lookup(id);
> > +		mem = id_to_memcg(id);
> >  		if (mem && !css_tryget(&mem->css))
> >  			mem = NULL;
> >  		rcu_read_unlock();
> > @@ -2231,7 +2244,7 @@ __mem_cgroup_commit_charge_swapin(struct
> > 
> >  		id = swap_cgroup_record(ent, 0);
> >  		rcu_read_lock();
> > -		memcg = mem_cgroup_lookup(id);
> > +		memcg = id_to_memcg(id);
> >  		if (memcg) {
> >  			/*
> >  			 * This recorded memcg can be obsolete one. So, avoid
> > @@ -2240,9 +2253,10 @@ __mem_cgroup_commit_charge_swapin(struct
> >  			if (!mem_cgroup_is_root(memcg))
> >  				res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> >  			mem_cgroup_swap_statistics(memcg, false);
> > +			rcu_read_unlock();
> >  			mem_cgroup_put(memcg);
> > -		}
> > -		rcu_read_unlock();
> > +		} else
> > +			rcu_read_unlock();
> >  	}
> >  	/*
> >  	 * At swapin, we may charge account against cgroup which has no tasks.
> > @@ -2495,7 +2509,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
> > 
> >  	id = swap_cgroup_record(ent, 0);
> >  	rcu_read_lock();
> > -	memcg = mem_cgroup_lookup(id);
> > +	memcg = id_to_memcg(id);
> >  	if (memcg) {
> >  		/*
> >  		 * We uncharge this because swap is freed.
> > @@ -2504,9 +2518,10 @@ void mem_cgroup_uncharge_swap(swp_entry_
> >  		if (!mem_cgroup_is_root(memcg))
> >  			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> >  		mem_cgroup_swap_statistics(memcg, false);
> > +		rcu_read_unlock();
> >  		mem_cgroup_put(memcg);
> > -	}
> > -	rcu_read_unlock();
> > +	} else
> > +		rcu_read_unlock();
> >  }
> > 
> >  /**
> > @@ -4010,6 +4025,9 @@ static struct mem_cgroup *mem_cgroup_all
> >  	struct mem_cgroup *mem;
> >  	int size = sizeof(struct mem_cgroup);
> > 
> > +	if (atomic_read(&mem_cgroup_num) == NR_MEMCG_GROUPS)
> > +		return NULL;
> > +
> >  	/* Can be very big if MAX_NUMNODES is very big */
> >  	if (size < PAGE_SIZE)
> >  		mem = kmalloc(size, GFP_KERNEL);
> > @@ -4020,6 +4038,7 @@ static struct mem_cgroup *mem_cgroup_all
> >  		return NULL;
> > 
> >  	memset(mem, 0, size);
> > +	mem->valid = 1;
> >  	mem->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
> >  	if (!mem->stat) {
> >  		if (size < PAGE_SIZE)
> > @@ -4049,6 +4068,7 @@ static void __mem_cgroup_free(struct mem
> >  	mem_cgroup_remove_from_trees(mem);
> >  	free_css_id(&mem_cgroup_subsys, &mem->css);
> > 
> > +	atomic_dec(&mem_cgroup_num);
> >  	for_each_node_state(node, N_POSSIBLE)
> >  		free_mem_cgroup_per_zone_info(mem, node);
> > 
> > @@ -4059,6 +4079,19 @@ static void __mem_cgroup_free(struct mem
> >  		vfree(mem);
> >  }
> > 
> > +static void mem_cgroup_free(struct mem_cgroup *mem)
> > +{
> > +	/* No more lookup */
> > +	mem->valid = 0;
> > +	rcu_assign_pointer(mem_cgroups[css_id(&mem->css)], NULL);
> > +	/*
> > +	 * Because we call vfree() etc...use synchronize_rcu() rather than
> > + 	 * call_rcu();
> > + 	 */
> > +	synchronize_rcu();
> > +	__mem_cgroup_free(mem);
> > +}
> > +
> >  static void mem_cgroup_get(struct mem_cgroup *mem)
> >  {
> >  	atomic_inc(&mem->refcnt);
> > @@ -4068,7 +4101,7 @@ static void __mem_cgroup_put(struct mem_
> >  {
> >  	if (atomic_sub_and_test(count, &mem->refcnt)) {
> >  		struct mem_cgroup *parent = parent_mem_cgroup(mem);
> > -		__mem_cgroup_free(mem);
> > +		mem_cgroup_free(mem);
> >  		if (parent)
> >  			mem_cgroup_put(parent);
> >  	}
> > @@ -4189,9 +4222,11 @@ mem_cgroup_create(struct cgroup_subsys *
> >  	atomic_set(&mem->refcnt, 1);
> >  	mem->move_charge_at_immigrate = 0;
> >  	mutex_init(&mem->thresholds_lock);
> > +	atomic_inc(&mem_cgroup_num);
> > +	register_memcg_id(mem);
> >  	return &mem->css;
> >  free_out:
> > -	__mem_cgroup_free(mem);
> > +	mem_cgroup_free(mem);
> >  	root_mem_cgroup = NULL;
> >  	return ERR_PTR(error);
> >  }
> > Index: mmotm-0811/init/Kconfig
> > ===================================================================
> > --- mmotm-0811.orig/init/Kconfig
> > +++ mmotm-0811/init/Kconfig
> > @@ -594,6 +594,16 @@ config CGROUP_MEM_RES_CTLR_SWAP
> >  	  Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
> >  	  size is 4096bytes, 512k per 1Gbytes of swap.
> > 
> > +config MEM_CGROUP_MAX_GROUPS
> > +	int "Maximum number of memory cgroups on a system"
> > +	range 1 65535
> > +	default 8192 if 64BIT
> > +	default 2048 if 32BIT
> > +	help
> > +	  Memory cgroup has limitation of the number of groups created.
> > +	  Please select your favorite value. The more you allow, the more
> > +	  memory(a pointer per group) will be consumed.
> > +
> 
> Looks good, quick thought - should we expost memcg->id to user space
> through a config file? I don't see any reason at this point, unless we
> do it for all controllers.
> 

I wonder....showing whether 2 interfaces as "path name" and "id" to users is a
good thing or not. Yes, it's convenient to developpers as me and you, others,
but I don't think it's useful to users, novice people.

I'd like to avoid showing show memcg->id to users as much as possible. I don't
want to say but memcg is enough complicated.


Thanks,
-Kame


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